The Prohibition era in America, which lasted for well over a decade and—inconceivable as it might be today—effectively banned the sale and production of booze in the United States, ended with the ratification of the 21st Amendment on Dec. 5, 1933.

Part detective story, part homage to the American immigrant experience and, ultimately, a tribute to the simple dignity of hard work, the documentary film Men at Lunchexamines the mysteries behind one of the most recognizable images of the 20th century: eleven men casually perched atop a steel beam hundreds of feet above Manhattan. For eight decades, from the time the photo was made in 1932 until brothers Seán and Eamonn Ó Cualáin came across a print of the photo in a pub in the west of Ireland in 2009, the picture was embraced as a stirring illustration of the creation of modern New York. Here, the picture seems to say, are the immigrants who built, by hand, the greatest skyline in the world. Here are the unsung heroes of Manhattan.

But as the Ó Cualáin brothers dug into the claim accompanying the picture they saw in that County Galway pub — namely, that the photo showed two local men, Sonny Glynn and Matty O’Shaughnessy, among the 11 on the girder in the sky — they discovered that little to nothing was truly known about the photograph itself. For instance, the picture is often misidentified as having been made atop the Empire State Building; it was actually taken during construction of Rockefeller Center. Even the photographer remains anonymous — for years the picture was wrongly credited, officially and unofficially, to the great Depression-era social reformer and photographer, Lewis Hine.

Today, four years after first encountering the photograph, and with their documentary film now in limited release, Seán (director) and Eamonn (producer) are still working to confirm the identities of most of the 11 men in the picture, even as they both have moved on to other film projects. Thus far, their inventive research into the skyscraper photo — including digital, 360-degree recreations of the image, beautifully rendered in the documentary — has uncovered the names of only two of the workers: Joe Eckner (third from left) and Joe Curtis (third from right).

Incredibly, their digging also brought to light another, previously unpublished portrait of the men on the beam on that long-ago September day — a photo, seen here (slide #2 above), of the 11 cheekily waving their hats.

Anyone with information about the photograph, or with documentation that might confirm the identities of any of the other nine men in the picture, is urged to contact Seán and Eamonn through the film’s Facebook page or via Twitter: @menatlunchfilm. Although, truth be told, Seán Ó Cualáin admits to an understandable ambivalence about the possible ramifications of their enduring investigation.

“Deep down,” he says, “I hope that the identities of all eleven men are not found. The mystery adds to the magic of the photo.”

A First Run release, Men at Lunch was financed by TG4, The Irish Film Board and The Broadcasting Authority of Ireland. It opens in New York at the Quad Cinema on Friday, Sept. 20.